mmc: dw_mmc: fix the wrong setting for UHS-DDR50 mode

When card is running with DDR mode, dwmmc needs to set DDR_REG bit at
UHS_REG register.
Before this patch, dwmmc controller doesn't consider this.
If this patch is not applied, CRC or other error shoulds be occurred.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>